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Homework Help: Angles between vectors

  1. Aug 9, 2008 #1
    ive got the formula down as angle=cos-1[(u.v)/(||U||||v||)

    with my u as 3,1,2 and v as -2,3,4 this gives me cos-1[(-6i+3j+8k)/root406) where do i go from here?
  2. jcsd
  3. Aug 9, 2008 #2


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    What is dot product supposed to give you? What did you get instead?
  4. Aug 9, 2008 #3
    that is what the dot product is ment to give me isnt it i just have no idea where to go after that to get the angle
  5. Aug 9, 2008 #4


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    No it isn't. The dot product is supposed to give you a scalar (ie a number), but you wrote that it gives you a vector. You can't very well take the arccosine of a vector can you?
  6. Aug 9, 2008 #5


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    [tex]a\vec{i}+ b\vec{j}+ c\vec{k}\cdot d\vec{i}+ e\vec{j}+ f\vec{k}[/tex]
    is defined as
    [tex]ad+ be+ cf[/tex]
    a number, NOT as
    [tex]ad\vec{i}+ be\vec{j}+ cf\vec{k}[/tex]
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